Meter system



NOV. 14, 1944. F. G, KELLY 2,362,562

METER SYSTEM Filed July 8, 1942 I ll] 9 E -E :Inventor .mimi q Frederick G- Kelly 21 '#11 8* Patented Nave METER SYSTEM Frederick G. Kelly. West Orange. N. J.. assigneeI to Thomas A. Edison, Incorporated, West Orange, N. J.. a corporation o! New Jersey Application July 8, 1942, Serial No. 450,222

13 Claims.

Y two electrical currents at least one of which is influenced by that quantity. The advantage, itself well known. oi a measurement by currentamplitude comparison between two currents is that, so long as the same source is used for producing both currents. the comparison may be kept i'ree of iniluence by variation in the voltage of the current source. l

By way oi specinc example. and without intending unexpressed limitations. I- have illustrated and most particularly described the invention as arranged for the measurement or a temperature-vsriable electrical resistance. In such a case that resistance may be exposed to an environment whose temperature is to be indicated. and the calibration ci' the meter system may be in terms o! that temperature.

It is an obiect of my invention to provide an improved meter system for the measurement ci an electrical quantity. and improvements in such systems.

Itisanobiectofmyinventiontoprovidean improved such system which involves the comparitzon o! the amplitudes of two electrical eurren It is an obieet to provide an improved meter constmction, and particularly an improved construction oi a ratiometer adapted for use in such a system as last described.

Such systems commonly comprise a bridge circuit in which the ratiometer is comprised and intowhichtherelseonnectedtheresistance or other element whose characteristic is to be measured. Itissncbiectoimyinventiontoprovide an improved bridge circuit for such systems, It is another object to provide an improved combination oi' bridge circuit with ratiometer therein.

l Other obiects are to provide a bridge circuit pcculiarly to an improved ratiometer. and to provide the improved ratiometer especially adapted to such a bridge circuit.

Among the more specinc obiects are to provide improved constructions of damping box, magnet and combinations oi' coils with damping box and with magnet: to provide an improved drift-torque arrangement: and to provide a generally simpliiied ratiomeier construction.

Other obiects are i improve the emciency of a ratiometer, to improve the emciency of the bridge adapted for use therewith, and to maintain the system tree oi' the influence oi temperature of the bridge and ratiometer components.

Other and allied objects will more fully appear from the following description-and the appended claims.

In the description ot my invention hereinafter set iorth reference is had to the accompanying drawing. in which:

Figure l is a front elevational view oi' a ratiometer according to my invention;

Figure 2 is a side elevational view of the same ratlometer, together with certain associated elements o! the meter system;

Figure 3 is a vertical cross-sectional view taken along the line I4 of Figure 2;

Figure 4 is a horizontal cross-sectional view taken substantially along the line 4 4 of Figure 3: i

Figure 5 is a vertical cross-sectional view taken along the line l-loi Figure i;

Figure o is a bottom plan view o! the ratiometer, with a section o! the external shield removed Figure 'l is a detail perspective view of the drifttorque magnet and the strap on which it is mounted: and

ligure 8 is a schematic diagram of a meter system in which the ratiometer ot earlier ilgures is incorporated.

Itwillbeunderstoodtbattheretereneestothe arrangement o! the ratiometer. as between vertical and horizontal (made to an orientation oi the instrument which brings its scale vertical) are arbitrary and made only because that orientation isacommonone-themovingsystemoitheinstrument being suitably balanced to provide for proper operation in any orientation Roierencebeinghsdtoliigureslthrough 8, the illustrated ratiometer may be seen to be assembled about a damping box ss its base-forming el t. I'his damping box comprises a shallow cupped member I and a cover member l.

both relatively thick and of highly conductive material such as copper. 'i'hese two members are held together by studs I passing therethrough and internally threaded bushings l tightened on the studs behind the member I-the passage ot the studs through this member occurring in the particularly wide side-wall regions la on each side of the cylindrical space t enclosed by the box. At the forward ends of the studs ltbereissecuredtheretoandextendsbetween themacross-member .intheoenteroiwhich thereisprovidedairontbearing 1. Inthe centerotthedamping-boxmemberithereisprovided a rear bearing 8. Between these bearings, and passing through a suitable oversized aperture 2a in the center oi the damping-box cover member 2, there is journalled the moving system of the instrument.

pThis moving system comprises a spindle IB on which there is secured, in a plane transverse of the spindle and within the space 3 enclosed by the damping box, a magnet II symmetrical about the spindle and extending therefrom to have its extremities closely adjacent the sidewalls of the space 3. The magnet II is prei'erably formed of one of the highly elcient magnet materials currently available, such as that commercially known as "Alnico." I have found it desirable that the magnet be slightly tapered in width, and substantially tapered in thickness, from its central to its extreme portions. On the spindle IB externally of the damping box, and for example slightly behind the iront bearing 1. there may be secured the usual tour-arm cross I2, of which three arms I2a may carry respective small longitudinally adjustable balancing weights Il-and of which the fourth arm I2b is extended. and curved rst forwardly and then upwardly, to form the needle lI4 of the instrument.

The damping-box cover member 2, already mentioned as being provided with the central aperture 2a to pass the spindle Il. is also provided with a narrow cut-through slot 2b extending from the apertime 2a to the periphery of the member, so that as the member 2 is slipped sidewardly into place in front of the member I the spindle i (meanwhile manually held in engagement with the bearing 8) may come into the aperture 2a. It is of course after this operation that the studs l and cross-member 6 and bearing I are assembled in place, to complete the journailing of the moving system.

While the structure as so far described is available for use in various types of meters, much of the particular arrangement ot further elements which I have illustrated is especially adapted for a ratiometer. Thus around the damping box, and in planes which extend from front to back of the instrument but are inclined from vertical (as illustrated. by 30 degrees to the left), there are supported by the damping box a pair oi coils I6. These coils may closely t the box, and may be positioned quite close to the spindle on the respective sides thereof. Accuratelyto establish the positions of these coils there may be provided on the damping box (e. g.. on both the members I and 2) rectangular projections I8, centered on a plane containing the axis of the spindle, and having faces forming abutments for the coils. One of the coils I6 may be pushed rightwardly from the left side of the damping box into tight contact with the lei'thand faces of the projections Il, while the other of these coils may be pushed leftwardly from the right side ci' the damping box into tight contact with the righthand faces of these projections. Around the damping box and coils III, and in planes which extend from front to back oi the instrument but are inclined from vertical opposite!!! to the inclination of the planes of coils Il (as illustrated, by 30 degree to the right) there are supported by the damping box a pair of coils Il. Accurately to establish the positions oi' these coils there may be provided on the damping box rectangular projections I1; these are similar in function and position to the projections Il above described. excepting that they function for coils I5 and that they are cn opposite sides oi the central plane of the instrument from projections I8.

It will of course be understood that the assembly of the coils I6 and I5 into at least approximate position is effected before the final assembly of the studs 4 and cross-member 6 and bearing 'I. After that iinal assembly the coils may be brought into nal position against the abutments, and secured in place either by cernenting to the damping box, or by somewhat resilient insulating pieces I9 stressed around the respectively adjacent studs to press against the outer coil surfaces, or by both these expedients.

To protect the magnet I I of the instrument from the spurious iniiuences of stray elds there is assembled around the damping box and coils a shield 20 of high-permeability and low-hysteresis material. This shield may be in the form of a cup, open at the front and slipped onto the assembly from the rear a little more than enough to fully surround the coils; the bushings l abovementioned may be of suitable length to be impinged against by the end of the shield 20 when the latter is in the desired position, the extreme threaded portions oi the studs l then penetrating through the end oi the shield and externally accommodating nuts 2| serving to clamp the shield in place. 1i desired. and as illustrated. there may be clamped between nuts 2| and the shield 20 the vertical portion of a bracket 22 by which the entire ratiometer proper `may be supported.

The bracket 22 Yhay hang down from a pair o! horizontal narrow plates 23 (between and slightly above which the top portion of the shield 2l may project), which plates forwardly may extend to approximately the plane of the crossmember 8; there they may be bent downwardly to form lugs 23a against which a disc 24 carrying a scale 26 for the instrument may be screwed. The lower central portion of the scale disc 24 may be cut away, to expose practically the full iront elevational extent ci the ratiometer proper and to permit the forwardly curved portion of the arm I2b to reach a plane in front of the scale before that arm is curved upwardly to form the needle I4.

The instrument so particularly described is especially adapted for the measurement c! the ratio between two currents respectively passed through the two pairs of coils It and It. The magnet II will align itself, both as to inclination of its magnetic axis (e. g., its longitudinal center line) and as to direction along that inclination. with the inclination and direction along and in which the net iield resulting from the two pairs oi' coiled is strongest. If current traverses only the pair o! coils I5, the net iield is the eld or that pair oi' coils and is strongest along a line inclined degrees to the left from vertical; the magnet I I will be inclined along such a line: the needle Il (assumlng the illustrated alignment Vof needle with magnet axis) will be similarly inclined: and, assuming the current flow to be in the appropriate one of the two possible directions. the direction of magnet and needle alignment along the Gil-degree inclination will be such as to bring the needle to the left extremity o! such a scale as scale 25- i. e., one which extends 60 degrees to the left and to the right from vertical. 0n the other hand if current traverses only the pair ot coils II, the net neld is the tield oi this pair of coils and is strongest along a line inclined 60 degrees to the right from vertical; the magnet I I will be inclined along such a line; the needle I 4 will be similarly inclined; and, assuming the current now to be in the appropriate one ci' the two possible directions,

the directionofmalnetandneedlealisnmentwill besuchastobrinstheneedletotherightextremity of scale il. With two currents 'simultaneously traversing the respecttive pairs of coils (each in the respective appropriate direction abovementioned) `the net iield is strongest along a line havins an inclination which lies within the range of plus-and-minus 60 degrees from vertical and whose exact value is dependent on the ratio or the currents; the magnet li will be inclined along such a line; and the needle il, similarly inclined,willpointtoanintermediatespotonscale il whose calibration will be a precise indication o! the ratito of the two currents respectively traversing the pairs of coils.

The structure which I have illustrated and described above is a very eillcient one in the perfomance of the function Just outlined. The sensitivity of magnet and needle to the current conditions to be measured is a function, among other things, oi' the ratio between the torque developed bythemovingsysteminresponsetoauniteid stimulus. and the mass of the moving system: this ratio is maintained at an especially hish value by the illustrated and described structure of the moving system, and particularly of the magnet itself. The sensitivity is further a function of the held stimulus. actually elective on the magnet, per unit coil current: this is kept high by the arrangement of the magnet relative to the coils-the coupling between them being unusually close for a well damped instrument. Not only is this coupling close as a generality, but it is maintained quite uniformly close throughout the very substantial length (e. g., 120 degrees) of scale. This relative uniformity of close coupling is particularly contributed to in this instrument by the distribution oi the horizontal legs of the coils (which legs the magnet poles most closely approach) about the axis of the instrument, and by the relationship between this distribution and the magnet poles. Thus it may be seen (best from Figure 5) that, both around the top and around the bottom of the instrument, between approximately 56 degrees to the left and 56 degrecs to the right of vertical there are only three breaks in the continuity of the peripheral spread of the horizontal coil less-aud these breaks are respectively only of the order oi' 18 degrees, 4 degrecs and 18 degrees. Bach magnet pole subieniis an angle about the axis ot approximately 45 degrees. From these relationships it may be shown that at any position within the IRO-degree scale at least ll desrees out of the 45 degrees-or approximately mths-of the magnet pole is radially aligned with horisontal-coil-leg portions. this beitng individually true for both poles of the masne 'rnc foregoing characteristics src achieves with sacrinceofdamping action.whichisvery Gmimtiypreservedinthisstructure. Thissetioniseifectedmponanytofthsmaglichbytheeddycurrentswhichthatmovsment induces in the damping box. The substantial continuityofenclosure'ofthemsanetbytbebox, notonlyaroundbutalsoinfrontofandbehind themegnehandtheconstructionoftheboxwith athicknessatleastoftbeorderomandiasillustrated and preferred) preferably' substantially greater than, the thickness of the bar magnet whichitenolosss,aswellastheciosespacingof the box from the magnet. are features prominently contributin to the effectiveness of the dampinsaction.

A meter system of which the ratiometer described above may form a part. and for which it is especially suited, is shown in Figure 8. By way oi' example, this system has been shown as one adapted for the measurement of the electrical resistance of a temperature-variable resistance element lli-for example, a resistance-thermometer bulb of the type disclosed and claimed in my co-pending application Serial No. 346,892, filed July 23, 1940, now issued as Patent No. 2,307,626, which type of bulb may be and normally is disposed remotely from the rest of the system. The measurement of the resistance of the bulb of other element Il serves as a measurement of the temperature of that element and hence of the ambient in which it is immersed; and the scale 2l of the ratiometer may be calibrated, for example, in terms of that temperature.

In Figure 8 it will be seen that the variable resistance element 30 and a resistance element 3i are connected in series with each other across a battery or other source of current lt; while also across the battery, and in series with each other, there are connected a resistance I! and a resistance $3. Since a circuit or cross-arm is to be connected from between 32 and Il to between il and Il, the arrangement is obviously a bridge-of which each of the four elements Il, Il, 8l, 3l may be termed a leg, and of which 39-8! may be termed one branch and ll-Il another branch. Physically, the resistances of the bridge may be conveniently positioned on the narrow plates 2l abovementioned, as has been indicated in the side elevational Figure 2.

According to my invention the cross-arm of the bridge is formed as a triangle or "delta," of which the two pairs oi' coils ii and Il respectively constitute the two sides, and a iixed resistance ll constitutes the base-the base being seriallyintcrposed between the two less of one branch (preferably n n) of the bridge, and the apex (or junction between the two pairs of coils) being connected to the Junction between the two legs oi' the other branch (preferably lI-ID of the bridse.

The bridge 'may be considered balanced" when the resistance of element 3|) is such as to cause the potential of the apex of the delta (e. g., the Junction between il and Il) to coincide with a potential midway between the potentials of the extremities of the base (e. g., of the extremities of resistance Il). Under this balanced condition the cross-current in the bridge, if considered as the algebraic sum of the two currents respectively traversing the two pairs of coils, would be aero-in that the two currents are equalV in magnitude, but iiow in opposite directions as between the two branches of the bridge. In the two pairs of coils. however, I so polarize them that each oi the two current directions Just mentioned constitutes for its pair t of coils the appropriate direction for the ratiometer action above outlined. The currents in the twopairsofooilsbeinlequllinthe balanced condition, the ratiometer then yields an intermediate reading (which would be a precisely midscale reading exoeptins for the slightly altering inuence or an eiiect hereinafter mentioned).

.Astberesistanceoftheelemepttl varies from um vciuc which causes me bricsc n i bcianced, the current traversins one pair of coils will increase and that traversing the other pair will decrease (their algebraic sum increasins poeitively or negatively from aero, while their arith metio sum remains substantially identical with the value it had in the balanced condition).

This obviously alters the ratio between the two currents; and the ratiometer needle Il will swing leftwardly or rightwardly from the abovementioned intermediate reading. according to whether the resistance lli has decreased or in creasedl in value. Upon a sumcient change of the resistance of element 3D the current in one of the pairs of coils (il if the change is a resistance reduction, I5 if a resistance increase) will be reduced to zero, while that in the other pair will have increased to substantially twice its balanced-condition value; under this condition the needle i4 will point to an extremity of the scale 25.

It is to be understood that upon a still greater variation of the resistance of element 3l beyond the condition last mentioned, although that one of the two coil currents which was zero in that condition now becomes negative (i. e., ilows in opposite direction to normal), the action of the system will continue smoothly, the needle Drogressing to ofi-scale readings. That this should occur may be best understood from the observation that the action of the system may be resolved into the production of a force tending to render the magnet vertical and proportional to the arithmetic sum of the two coil currents. and a force tending to render the magnet horizontal and proportional to the algebraic sum of the two currents; the ilrst sum is essentially constant. while the second obviously varies smoothly with variation of the resistance of element 30. The reason for ultimately terminating the scale Il (e. g., at 60 degrees from vertical) lies in the scale-extremity crowding which would occur in steeply increasing degree if the scale were made appreciably longer.

The coils IE, being in their inclined legs closer to the magnet. are slightly more effective than the coils I5, for any given current value presupposing an identical number of turns for the difierent coils. However, the relative effectiveness of the coils may be altered to any desired extent, or eliminated, by suitable differentiation of the numbers of turns in the coils of the respective pairs.

` Frequently however-as is the case with a typical resistance-thermometer bulb when employed for the element BIJ-there is some deviation of the variation of element 3|) from alinear relationship to the quantity (e. g., temperature) in terms ci' which the scale 25 is calibrated, tending to result in a compecting of one side of the scale (typically with the resistance-thermometer bulb, of the lowtemperature or lei'thand side) and expansion of to the other, accompanied by a shifting of what should be the central point of the scale. When one pair of coils is more effective than the other, there results a generally similar compacting of the side of the scale to which the axis of those coils inclines (for coils IB, a compacting of the righthand side of the scale) and expansion of the other: and this effect may be used to oii'set or approximately neutralize the abovemention'vd compacting eiect due to the element Il. Accordingly in a typical case I have preferred to employ a similar number of turns for both the coils I l and I8, in spite of the slightly more effective Positioning of the former.

In the foregoing discussion oi' the action of the system it has been assumed that the resistances of the two pairs of coils were identical. Whether or not the number of turns is similar for the two pairs. nevertheless the diilerence in average turnlength between the pairs tends to differentiate their resistances. While theoretically the net resistancediilerentiatins tendency might be neutralized by careful differentiation oi the sizes of wire employed for the coils oi the respective pairs, it is usually more desirable to employ an identical wire size for all coils, and to introduce a resistance '(e. g., 36, Figure 8) in series with the pair of coils of lower resistance, to render the total resistance of each side of the delta identical.

It is highly desirable that the system be free oi iniluence by the temperature of the ambient to which its elements (other, of course, than the element 3U) are exposed. It is of course possible tc employ for the resistance elements wire of negligible temperature coefficient, such as that commercially available under the trade-name Manganin, and this is preferably done for elements 3i and 32 and at least the major portion of 33. There remains. however, the coils i5 and IB themselves, which are of course preferably wound with copper wire, whose temperature coeiiicient is substantial (and preferably the small resistance 36 for symmetry of the delta. is of copper). I have round that it is possible to make the ratio of the currents in the two pairs of coils almost utterly independent oi temperature by the primary expedient of forming the resistance 34 (a relatively small resistance) partly of copper wire (section 34a, Figure 8) and partly of the negligible-coefilcient wire (section (Bibl-preferably coupled with the secondary expedient of forming a very small portion of the resistance 33 (section 33a) of copper wire.

I may mention that with a resistance-thermometer bulb (element whose resistance varies from 108 ohms at 50 degrees to 129 ohms at 100 degrees to 152 ohms at 150 degrees, and with coils (and resistance IB) such that the resistance oi each side of the delta ls approximately 320 ohms, I have satisfactorily employed resistances of the following approximate values: 3i, 700 ohms Manganin:Y 32, 121 ohms Manganin; 33a, 2l ohms copper; 33h, 679 ohms manganin; 38a, 9.3 ohms copper; and IIb, 6.7 ohms Manganin.

In the system as thus far described there is no provision for establishing any particular angular position of the magnet and needle in the absence of energization of the bridge by the current source. Frequently, however. it is desirable that there be established a bias, to some predetermined position, which will be eiiective in the absence of that energization. This may be done by a small auxiliary, or "drift-torque, magnet positioned for slight, but suiiicient, inuence on the main magnet Il. Such an auxiliary magnet I have illustrated as Il. I have found that a particularly favorable manner of mounting this auxiliary magnet is to provide an arm (e. g., 42) angularly adjustable about the axis oi pivoting of the main magnet, and to mount the auxiliary magnet on an outward portim oi this arm. In

- order to permit easy access to this arm tor adjustment. I prefer to pivot the arm (as at Il) to an upward central lug a formed from a strap Il, which extends horizontally in a vertical plane a little behindthe plane of the cross-member l and lust within the mouth of the shield. This strap may be formed with end lugs b extending first inwardly and then upwardly to be clamped to the iront oi' the studs l immediately behind the cross-member l.

It will be understood that the arm l! may be angular-ly adjusted so that the auxiliary magnet Il biases the main magnet Il to any desired anguiar position. It is oi' course true that the conetant bias exerted by the auxiliary magnet will be 'eective during periods of energization of the bridge (i. e., periods o! use) as well as in the absence of such energization. For any particular position of the auxiliary magnet, however, the eiiect oi' its constant bias during periods of energlzation may be compensated for to keep the meter indications accurate. by a suitable adjustment of the bridge resistances. Specifically, the horizontal component oi' this constant bias may be compensated i'or by an appropriate slight unbalance oi' the bridge (i. e.. slight revision of the value of any one oi the tlxed resistances JI. Il and 33) while the vertical component of this constant bias may be compensated for by an appropriate slight alteration of the value oi resistance.

While the foregoing description has been presented especially as to a meter system of the ratiometer type, and with particular reference to a specific such system. it will be appreciated that many of the aspects lof my invention are applicable to and useful in meter systems generally. and I therefore do not intend unexpressed limitations. The scope o1' my invention I undertake `to set forth in the appended claims.

I claim:

1. In a meter oi' the type comprising a magnet and a spindle on which said magnet is mounted for rotation, and coil means for creating a magnetic ileld ei'iective on said magnet: a damping box of material of high conductivity within which said magnet Yis rotatable. said box comprising base and cover members, said base member being provided with a bearing for said spindle, and said cover member being provided with a spindle-passing aperture and a cutthrough slot from said aperture to the periphery of said cover member.

2. In a ratiometer of the type comprising a magnet pivoted for rotation in response to a plurality of superimposed magnetic iields, and a plurality of coils for producing said fields:

a damping box within which said magnet is rotatable and around which said coils are positioned, said box being formed with a plurality of external abutments against which said coils may be pushed and which lform locaiizing means for said coils. t

3. In a ratiometer oi the. type comprising a magnet pivoted for rotation in response to a plurality of superimposedjmagnetic fields: the combination of a damping' box forming a relatively -close enclosure about said magnet and externally provided with aplurality oi' abutments: and a plurality of coils for producing said iields. surroundmg said box and positioned against and Ylocalised by said abilmente. A

l. In a meter system comprising a ratiometer having two coils at 'angles to 'each other. and an electrical bridge having two ybranches adapted for connection across a current source: a crossarm torlaaid bridge in the form oi' an electrical delta, Vthe base o! said delta comprising a resistance serially interposed in one of said brancheathe sides of said delta respectively comfprising laid coils, and the apex of said delta being -oonnected to a point in the other oi' said branches. i

5. In a meter system including a ratiometer: the combination o! a resistance to be measured and three nxed resistances electrically arranged in the form oi a bridge consisting of two branches adapted for connection across a current source; an added resistance serially interposed in a first of said branches; and two coils comprised in said ratiometer and connected from a common point in the second qi said bridge branches to the respective extremities oi said added resistance.

6. The combination according to claim 5, wherein said ilxed resistances are of lower temperature coemcient of resistance than that of said coils, and wherein said added resistance is of a temperature coemcient intermediate between those oi said ilxed resistances and of said coils.

7. In a meter system including a ratiometei` having a magnetic element pivoted for rotation in response to changes in the relative strength oi two superimposed magnetic fields, and an electrical bridge consisting of two branches adapted for connection across a current source, one of said branches comprising two ilxed resistances and the other of said branches comprising one fixed resistance and a resistance which is variable as a non-linear function of a condition to be measured: the combination of an added ilxed resistance serially interposed between the resistances of said one branch; a pair of coils for producing said magnetic ilelds, said coils being connected from a common point in said other branch to the respective extremities of said added resistance, and the effective spacing of one oi said coils with respect to said pivoted magnetic element beingdiierent than that of the other of said coils whereby said coils will have diii'erent effectiveness on said pivoted magnetic element to onset at least partially the non-linearity of variation of said variable resistance.

8. In a meter system including a ratiometer having a magnet pivoted ior rotation in response to changes in the relative strengths of two superimposed magnetic ficlds, and an electrical bridge consisting oi' two branches adapted for connection across a current source, one of said branches comprising two ilxed resistances and the other Ioi' said branches comprising a iixed resistance and a resistance which is variable as a nonlinear iunction of a condition to be measured: the combination o! an added resistance serially interposed between the said reslstances of said one branch: a pair of coils for producing said magnetic ilelds, said coils being connected from a .common point in said other branch to the respective extremities of said added resistance, and said coils being adapted to produce magnetic A i'ields of diil'erent eil'ectiveness on said magnet when the coils are current-energized to the same degree whereby to onset at least partially the non-linearity oi variation of said variable resistance.

` D.' In a meter system comprising a ratiometer having a magnetic element pivoted for rotation in response to changes in the relative strength of a plurality of superimposed magnetic fields, a plurality o! coils for producing said fields, and an electrical bridge having two branches adapted tor connection across ,a current source: a cross arm for said bridge in the iorm of an electrical delta.l the base oi said delta comprising a resistance serially interposed in one of said branches, the sides o! said delta respectively comprising said coils. and the apex of said delta being connected to a point in the other of said branches.

10. In a meter system comprising a ratiometer having a magnetic element pivoted for rotation in response to changes in the relative strength of two superimposed magnetic fields, and an electrical bridge having two branches adapted for connection across a current source: a cross arm for said bridge in the i'orm or an electrical delta,

the base oi said delta comprising a resistance serially interposed in one of said branches, the apex of said delta being connected to a point in the other of said branches, the sides of said delta respectively comprising said coils, and a compensating resistance for said system serially interposed in one of the sides of said delta.

ll. In a meter system including a ratiometer having a magnet pivoted to deect in response to changes in the relative strength of two superimposed magnetic elds, and a pair of coils for producing said fields: the combination of a bridge circuit for directing current from a current source through said coils and for causing a current increase in one of said coils and a current decrease in the other of said coils in response to an increase through a predetermined range of variation in the value of a resistance to be measured, said bridge consisting of two branches of which one branch serially includes said variable resistance and a fixed resistance and the other branch serially includes three fixed resistances, said coils being connected from the junction point of the resistanees of said one branch to therespective extremities of the middle resistance of said other branch` and a compensating resistance for said system connected between said branches of said bridge in series with said one coil.

l2. In a ratiometer system including a pair of coils i or producing superimposed magnetic fields: the combination of an electrical bridge for directing current from a voltage source through said coils and causing the currents in the coils to varil in opposite directions in response to a variation in the value of a resistance to be measured, said resistance being included in said bridge and said coils being connected between the branches of said bridge; a magnetic element pivoted to deflect in one direction and another from a mean position according to the relative effective values of the magnetic fields produced by said coils; and means to control the deflection characteristic of said pivoted magnet element in its range of deection to one side of its said mean position without' substantially ailecting its deection characteristic in its range to the other side oi' its mean position, said means comprising a resistance in said cross arm in series with one of said coils.

13. In a. meter system including a ratiometer: the combination of a resistance to be measured and three fixed resistances electrically arranged in the form of e. bridge consisting of two branches adapted for connection across a current source, said fixed resistances being each characterized as having a substantially negligible temperature coefficient; an added resistance serially interposed in the irst of said branches; and two coils comprised in said ratiometer and connected irom a common point in the second oi' said bridge branches to the respective extremities of said added resistance, the Wire of said coils being of a material having a substantial temperature coefficient, and said added resistance comprising two components comparable in magniture and made respectively of said material and of a material having a, substantially zero temperature coeiicient.

FREDERICK G. KELLY.

CERTIFICATE 0F CUtHECTION.

Patent No. 2,562,562.

FREDERICK G November 1li. 19M-t.

#ELLY It is mreby certified that error appears in the printed specification ci the above numbered patent requiring correction as follows Page 2, first column, line 6B, for degree read --degrees-; and second column, line 51|., for "coiledn read --Qoils--g page 5, first column, line it, for "rospoottivo read --respective--g "wi th read --without---g line lli., for ratito read --rat1o-; 11n 60, `for' page 5, first column, line 15, for "51, rood "-51, 52"; and that the said Letters Patent should be read with this oorrection therein that the ome may confom to the record o! mecano in tho Patent office.

signed m1 sealed una 1am dq or December, a. D. 19lt5.

(seal) Leslie .Frazer F11-st Assistant mmissionor of Patents.

the base oi said delta comprising a resistance serially interposed in one of said branches, the apex of said delta being connected to a point in the other of said branches, the sides of said delta respectively comprising said coils, and a compensating resistance for said system serially interposed in one of the sides of said delta.

ll. In a meter system including a ratiometer having a magnet pivoted to deect in response to changes in the relative strength of two superimposed magnetic elds, and a pair of coils for producing said fields: the combination of a bridge circuit for directing current from a current source through said coils and for causing a current increase in one of said coils and a current decrease in the other of said coils in response to an increase through a predetermined range of variation in the value of a resistance to be measured, said bridge consisting of two branches of which one branch serially includes said variable resistance and a fixed resistance and the other branch serially includes three fixed resistances, said coils being connected from the junction point of the resistanees of said one branch to therespective extremities of the middle resistance of said other branch` and a compensating resistance for said system connected between said branches of said bridge in series with said one coil.

l2. In a ratiometer system including a pair of coils i or producing superimposed magnetic fields: the combination of an electrical bridge for directing current from a voltage source through said coils and causing the currents in the coils to varil in opposite directions in response to a variation in the value of a resistance to be measured, said resistance being included in said bridge and said coils being connected between the branches of said bridge; a magnetic element pivoted to deflect in one direction and another from a mean position according to the relative effective values of the magnetic fields produced by said coils; and means to control the deflection characteristic of said pivoted magnet element in its range of deection to one side of its said mean position without' substantially ailecting its deection characteristic in its range to the other side oi' its mean position, said means comprising a resistance in said cross arm in series with one of said coils.

13. In a. meter system including a ratiometer: the combination of a resistance to be measured and three fixed resistances electrically arranged in the form of e. bridge consisting of two branches adapted for connection across a current source, said fixed resistances being each characterized as having a substantially negligible temperature coefficient; an added resistance serially interposed in the irst of said branches; and two coils comprised in said ratiometer and connected irom a common point in the second oi' said bridge branches to the respective extremities of said added resistance, the Wire of said coils being of a material having a substantial temperature coefficient, and said added resistance comprising two components comparable in magniture and made respectively of said material and of a material having a, substantially zero temperature coeiicient.

FREDERICK G. KELLY.

CERTIFICATE 0F CUtHECTION.

Patent No. 2,562,562.

FREDERICK G November 1li. 19M-t.

#ELLY It is mreby certified that error appears in the printed specification ci the above numbered patent requiring correction as follows Page 2, first column, line 6B, for degree read --degrees-; and second column, line 51|., for "coiledn read --Qoils--g page 5, first column, line it, for "rospoottivo read --respective--g "wi th read --without---g line lli., for ratito read --rat1o-; 11n 60, `for' page 5, first column, line 15, for "51, rood "-51, 52"; and that the said Letters Patent should be read with this oorrection therein that the ome may confom to the record o! mecano in tho Patent office.

signed m1 sealed una 1am dq or December, a. D. 19lt5.

(seal) Leslie .Frazer F11-st Assistant mmissionor of Patents. 

